The present invention generally relates to a Thin Film Transistor (TFT) tester and a TFT test methodology, and more particularly, to an apparatus and method for testing the operating characteristics of a TFT array using a non-contact voltage or current source.
The steps for manufacturing an organic light emitting diode (OLED) display (hereinafter referred to as an AMOLED display) of an active matrix are roughly divided into a ‘TFT array step’ of forming a TFT array for driving purposes, and a subsequent ‘cell step’ of forming the OLED on the array. At the stage at which the formation of the TFT array ends, the OLED which consists of a light emitting material, is not made on the TFT array substrate. Therefore, any circuit built therewith is inadequate to be used as a pixel circuit. Thus, only when the TFT array is formed, a drain or source of the TFT that is connected to an exposed pixel electrode is brought into an open state. Therefore, any operating current cannot pass (i.e., be turn on) through the TFT. As a result, at the stage where the TFT array step ends, it is impossible or very difficult to test the TFT electrical characteristics. Consequently, the characteristics of the AMOLED display required to complete the AMOLED display are usually obtained only after the cell step has concluded.
When the TFT array is tested following the TFT array step, any defective TFT array is prevented from being supplied to the cell step. As a result, the yield of the AMOLED display after the process has been completed is significantly higher, and a reduction in manufacturing costs can be anticipated. Since fluctuations of the TFT electrical characteristics seriously and adversely affect the image quality of the AMOLED display, there is a strong demand for performing the parametric tests at the TFT array stage.
By way of example, in Japanese Patent Application Laid-Open Nos. 2002-72918, 2002-108243, and 2002-123190, a method is disclosed in which current passes through a pixel electrode connected to the source or drain of the TFT used for driving and having an open state.
In Japanese Patent Application Laid-Open No. 2002-72918, a method is disclosed wherein the TFT array is immersed in an electrolytic solution to achieve electric conduction. Since the TFT array requires a full immersion in the solution, the method described is not actually used in a manufacturing line.
In Japanese Patent Application Laid-Open No. 2002-108243, a method is described in which the pixel electrode is tested before patterning. The method described also discloses forming and testing a conductive film on the pixel electrode, and the conductive film used for testing is removed after testing has been completed. However, the conductive film is stacked and formed on the TFT array and brought in close contact with the pixel electrode. Therefore, there is a danger that the TFT array will be damaged when forming the conductive film when tested on the TFT array and/or when removing the film. Therefore, there is little likelihood that such a method will actually be employed in a manufacturing processing line.
In Japanese Patent Application Laid-Open No. 2002-123190, a method is described in which an electrode is positioned in the upper portion of the TFT array substrate, and air between the substrate and the electrode is irradiated with an electromagnetic wave (soft X-ray) to ionize the air, such that electric conduction is achieved. However, during this process, the TFT array substrate is also irradiated with X-rays. Therefore, when too much current passes, the exposure of X-ray on the TFT element increases, and there is a possibility of the element breaking. The tester using soft X-ray requires a certain level of caution when handling it, special equipment is required to prevent exposure to the surrounding environment or operator, and further, it is not easy to perform. Therefore, there is little likelihood that this approach could be used in a manufacturing process.